An internal combustion engine has been developed with high operating efficiency obtained by means of an improved cooling system in which circulation of the engine coolant through the engine is carried out by using a water pump of which the water outlet is communicated with a water jacket of the engine and the engine coolant inlet is communicated with an outlet tank of the radiator. A bypass, which allows the engine coolant to reenter the water pump, is provided between the thermostat valve and the water inlet of the water pump. Such a cooling system is disclosed in, for example, Japanese Unexamined Utility Model Publication No. 57-30,322 entitled "Engine Cooling System For Engines" laid open Feb. 17, 1982.
To increase fuel mileage or decrease fuel consumption in low engine load range, it was taught that a high range thermostat that starts opening near 82.degree. C. (179.6.degree. F.) and is fully open around 100.degree. C. (212.degree. F.) can be used. When the high range pellet thermostat is used, the engine coolant flows through the bypass being kept away from passing the radiator below an engine coolant temperature of around 100.degree. C., whereby the engine operating temperature is maintained high, so as to contribute to an increase of fuel mileage in the low engine load range.
Such a cooling system is, however, apt to keep the engine coolant away from passing through the radiator when the engine load rapidly changes from low to high. Circulating a high temperature of engine coolant through the engine results in a lowering in cooling efficiency and decreases fuel mileage. On the other hand, if the engine load rapidly changes from high to low, the engine coolant becomes too cool before the thermostat valve fully closes, so as to increase fuel mileage consumption and cause a problem of emission control.